1. Field of the Invention
The present invention relates to an optical fiber grating (optical component) having an optical fiber and a Bragg grating provided in the core region of the optical fiber along its longitudinal direction and a method of manufacturing the same.
2. Related Background Art
In recent years, optical communication system configurations have advanced along with the recent developments of optical fiber communication techniques to realize sophisticated networks and achieve signal wavelength multiplexing. In these optical communication systems, the importance of optical circuit elements (optical components) is increasing.
A fiber type element as a general example of the optical circuit elements has advantages in that it is compact and has a small insertion loss and it can be easily connected to an optical fiber serving as a transmission line. An example of such a fiber type optical component is a fiber type filter. As is known well, when an ultraviolet ray is irradiated on glass doped with germanium oxide (GeO.sub.2), the refractive index changes in the irradiated portion.
In recent years, research and development of an optical fiber grating having a Bragg grating formed in the core region of an optical fiber have been made as an example of a fiber type filter using a photoinduced refractive index change. In this specification, an optical fiber grating is defined as an optical component having at least an optical fiber comprising a core region having a predetermined refractive index and doped with GeO.sub.2 and a cladding region provided around the core region and having a lower refractive index than the core region, and a Bragg grating formed in the core region of the optical fiber along its longitudinal direction. The grating is defined as a region in which the refractive index of the core region periodically changes along the longitudinal direction of the core region irradiated with an ultraviolet ray, or the like.
More specifically, the optical fiber grating has a function of reflecting a light component having a specific wavelength (to be referred to as a reflection wavelength of grating hereinafter) of light propagating along the optical fiber and transmitting the remaining light component (i.e., a light component having a wavelength shifted from the reflection wavelength of grating). The reflection wavelength of grating is determined by the pitch of a refractive index change induced in the core region. A method of forming a photoinduced grating in an optical fiber upon irradiation of an ultraviolet ray has an advantage of high productivity.
In such an optical fiber grating, its reflectance R serves as an important characteristic factor. The reflectance R depends on the length of the grating (i.e., the length of a region in which the refractive index of the core region periodically changes along the longitudinal direction of the core region) and the amount of the photoinduced refractive index change. This relation is expressed as: EQU R=tanh.sup.2 (L.pi..DELTA.n/.lambda..sub.R)
where
R: reflectance
L: length of grating
.DELTA.n: amount of photoinduced refractive index change
.lambda..sub.R : Bragg wavelength.